Narrow surface corrugated grating

ABSTRACT

Narrow surface corrugated gratings for integrated optical components and their method of manufacture. An embodiment includes a grating having a width narrower than a width of the waveguide on which the grating is formed. In accordance with certain embodiments of the present invention, masked photolithography is employed to form narrowed gratings having a desired grating strength. In an embodiment, an optical cavity of a laser is formed with a reflector grating having a width narrower than a width of the waveguide. In another embodiment an integrated optical communication system includes one or more narrow surface corrugated gratings.

TECHNICAL FIELD

Embodiments of the present invention are in the field of integratedoptical components (IOC) and more specifically pertain to surfacecorrugated gratings.

BACKGROUND

Communications networks continue to grow in breadth of coverage and datadensity. An important enabling technology of this continued growth isincreased integration of optical (photonic) components. For example,metropolitan area networks and wide area networks are now being deployedwith wavelength division multiplexing (WDM) which add/drop channelsusing wavelength selective filters integrated onto silicon, or othersemiconductor, substrates using very large scale integration (VLSI)manufacturing techniques.

In optical communication there are many applications in addition towave-length selective filters which to at least some extent utilize aBragg grating, such as lasers (e.g., distributed Bragg reflector (DBR)laser or distributed feedback (DFB) lasers), grating-assisted couplers,and dispersion compensators to name only a few. One type of integratedBragg grating, typically referred to a “corrugated grating” is formed byphysically corrugating a surface of a waveguide (e.g., planar orrib/ridge waveguides) patterned into a thin film over a substrate. For afirst-order corrugated grating to be operative at the 1550 nmwavelength, the grating period, or “tooth” pitch is between about 200 nmand 250 nm. This relatively small feature pitch leaves little latitudefor tuning the grating strength (κ) using VLSI manufacturing techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example,and not limitation, in the figures of the accompanying drawings, inwhich:

FIG. 1A illustrates an isometric view of a corrugated surface grating,in accordance with an embodiment;

FIG. 1B illustrates an isometric view of a corrugated surface grating,in accordance with an embodiment;

FIG. 2 illustrates a graph of grating strength as a function of gratingwidth for a constant waveguide width, in accordance with embodiments;

FIG. 3 illustrates a plan view of a corrugated surface grating, inaccordance with an embodiment;

FIG. 4A illustrates a plan view of a corrugated surface grating, inaccordance with an embodiment;

FIG. 4B illustrates a plan view of a corrugated surface grating, inaccordance with an embodiment;

FIGS. 5 is a flow diagram of a method of forming a corrugated surfacegrating, in accordance with an embodiment;

FIG. 6A illustrates a cross-sectional view of a photonic deviceincluding a pair of grating mirrors in passive waveguide regionsadjacent to an active waveguide region, in accordance with anembodiment;

FIG. 6B illustrates a cross-sectional view of a photonic deviceincluding a pair of grating mirrors in passive waveguide regions and anactive waveguide region, in accordance with an embodiment; and

FIG. 7 depicts an optical communication system including a plurality ofphotonic devices integrated together on a common substrate.

DETAILED DESCRIPTION

Embodiments of narrow surface corrugated gratings, their manufacture andapplication in integrated optical components are described herein withreference to figures. As referred to herein, a surface corrugatedgrating is “narrow” where the grating width is narrower than the widthof waveguide on which the grating is formed.

Particular embodiments described herein may be practiced without one ormore of these specific details, or in combination with other knownmethods, materials, and apparatuses. For example, while a grating mirroris described in the context of silicon-based DBR and DFB lasers, anarrow surface corrugated grating and the techniques described hereinmay be readily adapted to other integrated optical components, such as,but not limited to optical add/drop filters, signal conditioners, etc.In the following description, numerous specific details are set forth,such as specific materials, dimensions and material parameters etc. toprovide a thorough understanding of embodiments of the presentinvention. In other instances, well-known optical design and VLSIfabrication techniques have not been described in particular detail toavoid unnecessarily obscuring the present invention. Referencethroughout this specification to “an embodiment” means that a particularfeature, structure, material, or characteristic described in connectionwith the embodiment is included in at least one embodiment of theinvention. Thus, appearances of the phrase “in an embodiment” in variousplaces throughout this specification are not necessarily referring tothe same embodiment of the invention. Furthermore, the particularfeatures, structures, materials, or characteristics may be combined inany suitable manner in one or more embodiments. It should also beunderstood that specific embodiments may be combined where not mutuallyexclusive.

The terms “over,” “under,” “between,” and “on” as used herein refer to arelative position of one member with respect to other members. As such,for example, one member disposed over or under another member may bedirectly in contact with the other member or may have one or moreintervening members. Moreover, one member disposed between members maybe directly in contact with the two members or may have one or moreintervening members. In contrast, a first member “on” a second member isin intimate contact with that second member. Additionally, the relativeposition of one member with respect to other members is providedassuming operations are performed relative to a substrate common to themembers without consideration of the absolute orientation of thesubstrate or members.

Referring to FIG. 1A, an isometric view of an exemplary narrow surfacecorrugated grating 100 is depicted. The narrow surface corrugatedgrating 100 includes a grating 115 formed by corrugating a portion ofwaveguide 110 over a substrate 105. The waveguide 110 has a top surface111 which has a width W_(WG) as defined by a first waveguide sidewall112 and a second waveguide sidewall 113 further defining the rib orridge height H_(R). Because the waveguide sidewalls 112 and 113 may notbe precisely vertical (i.e. orthogonal to the top surface 111), thewidth W_(WG) of the top surface 111 is used herein as the waveguidewidth. In a particular embodiment, the width W_(WG) is betweenapproximately 0.3 and 2.5 μm and the rib height H_(R) is betweenapproximately 0.2 μm and 2 μm. Typically, a grating will be formed in aportion or region of a waveguide which has a constant waveguide width,as depicted in FIG. 1A. However, in alternative embodiments, a narrowsurface corrugated grating is formed in a tapered portion of awaveguide.

Narrow corrugated surface gratings are generally applicable to anymaterial system known in the art for corrugated surface gratings. Forexample, substrate 105 may be composed of any material suitable forintegrated optical component fabrication. In one embodiment, substrate105 is a bulk substrate composed of a single crystal of a material whichmay include, but is not limited to, silicon or a III-V compoundsemiconductor material, such as indium phosphide (InP). In anotherembodiment, substrate 105 includes a bulk layer with a top epitaxiallayer formed over the bulk layer. In a specific embodiment, the bulklayer is composed of a single crystalline material which may include,but is not limited to, silicon or a III-V compound semiconductormaterial, while the top epitaxial layer is composed of a singlecrystalline layer which may include, but is not limited to, silicon or aIII-V compound semiconductor material. In another embodiment, the topepitaxial layer is separated from the bulk layer by an interveninginsulator layer, such as silicon dioxide, silicon nitride and siliconoxy-nitride (e.g. to form a silicon-on-insulator substrate). Thewaveguide 110 may be, for example, any of those materials described ascandidates for the substrate 105 or may be others known in the art, suchas polymers (SU-8, etc.).

The grating 115 includes a plurality of grooves including grooves G₁,G₂, G₃ through G_(n) formed into the waveguide top surface 111 along agrating length, L_(G). The grooves G₁-G_(n) corrugate the top surface111 of the waveguide 110 resulting in periodic arrangement of groovesand “teeth” or “ridges” between the grooves forming a Bragg grating tomodulate the index of refraction in a portion of the waveguide 110. Thegrooves G₁-G_(n) have a grating pitch or period, P_(G) which, dependingon embodiment, can be uniform or graded, and either localized ordistributed in a superstructure. Certain embodiments of narrow gratingsmay also be tilted such that the grooves G₁-G_(n) are tilted from theorientation depicted in FIG. 1A (i.e., grooves are non-orthogonal to thelength of the waveguide). The grooves G₁-G_(n) have a corrugation depthD_(G) which is generally significantly less than the rib height H_(R),with deeper groove depths increasing grating strength. In one embodimentwhere the grating is formed in a silicon waveguide having a width ofbetween approximately 1-1.5 μm and a rib height H_(R) of approximately0.5 μm the depth D_(G) is between approximately 10-300 nm.

As further shown in FIG. 1A, the grooves G₁-G_(n) have a lengthdimension defining a grating duty cycle, DC based on the grating period,P_(G). The grating duty cycle is the ratio of the space between grooves(i.e. ridge length) and the grating period and is therefore a functionof the smallest groove spacing and the smallest groove dimension (i.e.groove length) achievable for a given patterning method. The gratingduty cycle decreases as the groove to tooth length ratio becomes largerfor a particular grating pitch P_(G). For example, a grating duty cycleis 50% where a groove has a length equal to that of a tooth and for agrating pitch of 240 nm and a 150 nm groove length (90 nm tooth)provides a duty cycle of 37.5% while a 90 nm groove length provides aduty cycle of 62.5%. In one particular embodiment where a grating isformed in a silicon waveguide having a width of approximately 1-1.5 μmand a rib height H_(R) of approximately 0.5 μm, the grating corrugationdepth D_(G) is approximately 10-300 nm while the grating length L_(G) isapproximately 1-200 μm and the plurality of grooves has a pitch (i.e.,grating period P_(G)) of approximately 190-250 nm. This exemplaryembodiment is well suited for integrated optical waveguide applicationsin telecommunications utilizing a 1550 nm nominal wavelength.

In embodiments, at least one of the grooves G₁-G_(n) has a widthnarrower than the waveguide width. In the exemplary narrow corrugatedsurface grating 100, each of the plurality of grooves has a width W_(G)narrower than the waveguide width, W_(WG). Generally, to achieve areduction in grating strength, substantially all of the grooves G₁-G_(n)are to have a width narrower than that of the waveguide. However, one ormore of the grooves G₁-G_(n) may have a width W_(G) equal to thewaveguide width without departing from the spirit of a narrow corrugatedsurface grating as long as the number of the grooves G₁-G_(n) which arenarrower than the waveguide width is sufficient to achieve anappreciable reduction in grating strength. In particular embodimentstherefore, at least 95% of the grooves in a narrow grating are of awidth narrower than the waveguide. In further embodiments, the width ofa majority of the grooves is narrower than the width of the waveguide byapproximately the same amount (i.e. a majority have a same width). In aparticular embodiment, as depicted in FIGS. 1A and 1B, all (orsubstantially all) of the grating grooves G₁-G_(n) or grating teethT₁-T_(n), respectively, have approximately the same narrowed width alongthe grating length L_(G).

In embodiments, a narrow surface corrugated grating includes at leastone groove of a width less than or equal to 90% of the waveguide width.Thus, for an exemplary waveguide that is approximately 1.5 μm wide, anarrow surface corrugated grating would have a grating width less thanabout 1.35 μm. In a further embodiment, a narrow surface corrugatedgrating includes at least one groove of a width at least 5% of thewaveguide width but no greater than 90% of the waveguide width W_(WG).Thus, for an exemplary waveguide that is approximately 1.5 μm wide, anarrow surface corrugated grating would have a grating width betweenabout 75 nm and 1.35 μm.

Narrowing the grating width to be less than the waveguide width allowsthe grating strength κ to be controlled. Hence, the reflectivity andbandwidth may also be controlled as a function of grating width toprovide an additional degree of freedom in the design and formation of asurface corrugated grating. The reflectivity R of the grating isapproximated in Equation 1, as

R≅tan h ²(κL),   (1)

where L is the grating length (e.g., L_(G) in FIG. 1A). Experimental andfield simulation data for a narrow surface corrugated grating aredepicted in FIG. 2. As shown, the grating strength κ for a gratingpatterned on a 1.5 μm wide waveguide is modeled as a function of gratingwidth. The grating strength was experimentally determined for gratingwidths between 1.3 μm and 0.3 μm, with the grating strength reduced from320 cm⁻¹ for the 1.3 μm grating width to 78 cm⁻¹ for the 0.3 μm gratingwidth.

Referring back to FIG. 1A, setting the width of the grating to controlthe grating strength κ has the benefit of being independent of thegrating pitch P_(G) or grating depth D_(G) which are each dimensionallymuch smaller than the waveguide width W_(WG). Thus, the resolution ofthe grating patterning method does not pose the same limitations as itdoes if the grating strength is modulated by varying the grating dutycycle, for example. In certain applications, grating depth may be sosmall, potentially on the order of tens of nanometers, that the grooveetch becomes quite difficult to controllably manufacture. Narrowedgrating grooves, having reduced grating strength relative to full widthgrooves, however can be etched to a relatively greater depth, forexample 2-3 times deeper, to achieve a particular grating strength.Thus, additional groove etch process latitude and controllability isavailable with the narrow surface corrugated gratings described herein.Also, because grating depth is typically difficult to vary betweengratings on a same substrate, a relatively larger grating depth may beprovide for all gratings to achieve a first grating of maximum gratingstrength which can then be reduced on a grating specific basis bynarrowing the grating width for other gratings on the substrate. Forthese reasons, a large range of grating strength κ may be achieved by anarrow corrugated surface grating for a given grating length.

As further depicted in FIG. 1A, the grating 115 has a longitudinalcenterline approximately aligned with the longitudinal centerline of thewaveguide 110, leaving spaces S₁ and S₂ between the grooves and thewaveguide sidewalls 112 and 113. In the exemplary embodiment depicted,the dimension of S₁ is approximately equal to that of S₂ as a result ofthe centerline alignment and the constant grating width along thegrating length, but it is understood that a certain amount ofmisalignment between the longitudinal centerlines of the waveguide andgrating is to be expected as a function of the method of manufacturingemployed and the tolerances of the particular equipment carrying outthat method. Generally, it has been found that the sensitivity tomisalignment between the longitudinal centerlines of the grating andwaveguide is inversely proportional to the grating width W_(G).

FIG. 1B depicts a embodiment of a narrow surface corrugated grating asan alternative to the embodiment illustrated in FIG. 1A. In thisalternative embodiment, the narrow surface corrugated grating 101includes grating teeth, T₁-T_(n) having a width narrower than thewaveguide width W_(WG). As depicted, a contiguous portion of waveguideis removed to define the grating teeth T₁-T_(n) and thereby form thespaces S₁ and S₂ along the entire grating length L_(G). The spaces S₁and S₂ are then to be substantially filled with a suitable claddingmaterial along with the grooves between the teeth T₁-T_(n), just as thegrooves G₁-G_(n) are for the embodiment of FIG. 1A. Depending on thegrating pitch P_(G) and duty cycle DC, a cladding fill around thefree-standing grating teeth T₁-T_(n) may be superior to a cladding fillin the grooves G₁-G_(n) of FIG. 1A. The remaining features of the narrowsurface corrugated grating 101 depicted in FIG. 1B are substantially thesame as those of the narrow surface corrugated grating 100 depicted inFIG. 1A.

In a further embodiment, the grating width varies as a function ofgrating length such that the widths of each groove in the plurality ofgrooves making up the surface corrugated grating are not all equal. Sucha grating architecture allows the grating strength to be varied alongthe grating length to provide an apodized narrow surface-corrugatedgrating capable of reducing side-lobe strength. Such grating widthapodization can be used to change the reflectivity and bandwidth of thegrating to reduce insertion loss and/or smooth the transmission spectra.Modulation of the grating width W_(G) may provide apodization of anytype, such as, but not limited to Gaussian and raised-cosine.

FIG. 3 illustrates a plan view of one embodiment of a grating widthapodized surface-corrugated grating 300. As shown, a grating 315 isformed on a waveguide 310 over a substrate 305. The grating width W_(G)varies from W_(G, MIN) to W_(G), MAX along the grating length L_(G).Although the grating width W_(G) of every portion of the grating is lessthan the waveguide width W_(WG) in the exemplary embodiment depicted inFIG. 3, in other embodiments some subset of the grooves making up thegrating may have a width equal to the waveguide width W_(WG).

In the embodiment depicted in FIG. 3, the grating pitch and duty cycleDC remain constant over the grating length L_(G). In other embodiments,however, both duty cycle DC and grating width W_(G) may be varied overthe grating length L_(G) to potentially provide greatercoupling-coefficient variation that would be possible through eitherwidth or duty cycle modulation may achieve alone. FIG. 4A depicts awaveguide 410, a portion of which contains the grating 415. The grating415 has a constant grating pitch P_(G) while the width of the gratingvaries from W_(G,1) to W_(G,2) to W_(G,3) along the grating length. Thegrating duty cycle DC also varies from DC₁ to DC₂ to DC₃ along thegrating length.

In another embodiment, a narrow grating width is utilized in conjunctionwith a modulated grating period along the length of the grating (e.g.,chirped narrow surface corrugated grating). For example, FIG. 4B depictsa grating 415 having a grating width which varies from W_(G,1) toW_(G,n)while the grating period varies from P_(G,1) to P_(G,2). In otherembodiments, the grating period is graded in any manner known in the artto control the reflected/transmitted spectrum (e.g., broadening thereflected spectrum) in conjunction with the grating having a fixed widththat is less than the width of a waveguide.

In embodiments, the narrow surface corrugated grating described inreference to FIGS. 1-4B is patterned with masked photolithography.Masked photolithography has many advantages over typical methods ofpatterning gratings, such as holographic (interference) and electronbeam patterning techniques. For example, masked photolithography has theadvantage of being a widely employed VLSI manufacturing process and thisform of lithography offers the ability to form a grating narrower than atypical waveguide along with adequate alignment to the waveguide.Holography techniques, in contrast, generally rely on patterning aninterference pattern across very large area (e.g., an entire substrate),while Ebeam writing is a relatively slow process with relatively pooreralignment capability. Use of masked photolithography also enablesgrating strength to be tuned to different values across a samesubstrate, thereby allowing grating filters or mirrors to be tuned fordifferent wavelengths between optical components on a same substrate oreven between two different portions of the same optical component.

FIG. 5 depicts a flow diagram of an exemplary masked photolithographymethod 500 of forming a narrow surface corrugated grating. The method500 begins with provision of a substrate at operation 501, such as anyof those described for substrate 105 in FIG. 1A. For example, in oneembodiment a silicon-on-insulator substrate is provided.

Next, at operation 505, a grating having a particular length, period andwidth is patterned in a material layer using a first photomask pattern.The patterning operation 505 may include any convention photolithographyprocess known in the art. However, in one embodiment 193 nm lithographyis employed. Steppers at the 193 nm lithography node are capable ofprinting sufficient minimum feature sizes (e.g., on the order of 90 nm)to print a grating with a sufficient period (e.g., on the order of 200nm to 250 nm) for an optical waveguide formed in the silicon anddesigned for the nominal 1550 nm wavelength utilized in opticalcommunication.

Thus, in one embodiment, at operation 505, a SOI substrate is coatedwith a photosensitive layer and exposed with 193 nm wavelengthelectromagnetic energy to print into the photosensitive layer a gratingpattern based on the first photomask. Depending on the embodiment, thegrating pattern may be either one which provides resist openings wherenarrowed grating grooves are to be etched or resist pillars wherenarrowed grating teeth are to formed in the substrate. In either case,the first photomask may be a bright field mask (masking only a portionof the grating and an area slightly larger than the waveguide) or adarkfield mask (exposing only a portion of the grating). Thephotosensitive layer is then developed into an etch mask and the patterntransferred into an underlying intervening hardmask layer or directlyinto the waveguide layer using any etch process, wet or dry, known inthe art. For example, a top silicon layer of an SOI substrate may beetched to transfer the grating pattern of the etch mask into the siliconlayer. In some embodiments, a double patterning method is employed toreduce the pitch of the grating below that of the first photomask. Anyconventional double patterning method known in the VLSI arts may beemployed. In one exemplary embodiment, the exposed pattern istransferred into an underlying etch mask layer, a spacer is formed oneither side of the mask layer, the mask layer removed, and the spacerthen used as a half pitch mask for the etching a grating into thesubstrate (e.g., top silicon layer of a SOI substrate).

In the exemplary masked photolithography method 500, a waveguide is thenphotolithographically patterned with a second photomask at operation 510after the grating pattern is formed in a layer over, on, or in thesubstrate (e.g., top silicon layer of an SOI substrate). The secondphotomask is aligned to the grating pattern to have the waveguidepattern encompass the grating (i.e., waveguide pattern is both longerand also wider than at least some portion of the grating). In oneembodiment 193 nm lithography is employed to pattern the waveguide.Steppers at the 193 nm lithography node are capable of aligning thewaveguide photomask to the grating photomask with sufficiently smallmisregistration (e.g., on the order of 100 nm) that a wide range ofgrating widths can utilized for purposes of controlling the gratingstrength. In an alternative embodiment, the waveguide may be firstpatterned and the grating pattern subsequently aligned to the waveguidepattern and printed as the second photomask pattern. However, it isadvantageous to pattern the grating prior to patterning the waveguidebecause the relatively smaller dimensions of the grating are more easilyachieved with a flatter substrate surface.

After patterning of both the grating and the waveguide, method 500 iscomplete and any further conventional processing of the waveguide and/orgrating may then be performed. For example, a cladding may be formedaround the waveguide and grating, filling the grooves and/orencapsulating the teeth of the grating. Any clad layer material thatprovides sufficient index contrast, as dependent on the material systemutilized for the waveguide, may be employed for the cladding. In oneexemplary SOI embodiment, the silicon waveguide uses a silicon dioxideclad layer to cover the waveguide and fill the grating grooves. In analternated SOI embodiment, SU-8 is utilized as cladding on a siliconwaveguide.

The narrow surface corrugated gratings and methods to form such gratingsmay be applied to a number of optical applications, such as, but notlimited to, integrated optical grating filters and integrated opticalgrating mirrors. Integrated optical grating mirrors may be morespecifically utilized to form optical cavities of DFB and DBR lasers.

FIG. 6A illustrates a cross-sectional view showing one embodiment of anelectrically pumped hybrid semiconductor evanescent laser employing aleast one narrow surface corrugated grating. The depicted cross-sectionis taken down the longitudinal centerline of the hybrid semiconductorevanescent laser and gratings. As shown, DBR laser 601 is integrated inan SOI substrate including a single crystalline semiconductor layer 603with a buried oxide layer 629 disposed between the semiconductor layer603 and a substrate layer 631. In one example, the semiconductor layer603 and the substrate layer 631 are made of passive silicon. As shown,an optical waveguide 605 is disposed in the semiconductor layer 603through which an optical beam 619 is directed. In the exampleillustrated in FIG. 6A, optical waveguide 605 is a rib waveguide, stripwaveguide, or the like. An optical cavity 622 forms an active portion ofthe waveguide between grating reflectors 607 and 609 in a passiveportion of the optical waveguide 605 adjacent to either end of theoptical cavity 622. As shown in FIG. 6A, reflectors 607 and 609 areBragg grating reflectors and in a particular embodiment at least one ofthe reflectors 607 and 609 is a narrow surface corrugated grating havinga grating width less than the width of a passive portion of the opticalwaveguide 605, such as any of those previously described herein.

A III-V gain medium 623 is bonded to or epitaxially grown on “top” ofthe semiconductor layer 603 across the “top” of and adjoining opticalwaveguide 605 to provide a gain medium-semiconductor material interface633. Interface 633 extends along the optical waveguide 605 parallel tothe direction of propagation of the optical beam 619. In one example,the gain-medium-semiconductor material interface 633 is an evanescentcoupling interface that may include a bonding interface between theactive gain medium material 623 and the semiconductor layer 603 ofoptical waveguide 605. For instance, such a bonding interface mayinclude a thin silicon dioxide layer or other suitable bonding interfacematerial. As one example, the gain medium material 623 is an activeIII-V gain medium and there is an evanescent optical coupling at thegain medium-semiconductor material interface 633 between the opticalwaveguide 605 and the gain medium material 623. Depending on thewaveguide dimensions of optical waveguide 605, a part of the opticalmode of optical beam 619 is inside the III-V gain medium material 623and a part of the optical mode of optical beam 619 is inside the opticalwaveguide 605. The gain medium material 623 may be electrically pumpedto generate light in the optical cavity 622.

In embodiments, the gain medium material 623 is active semiconductormaterial such and is III-V semiconductor bar including III-Vsemiconductor materials such as InP, AlGaInAs, InGaAs, and/orInP/InGaAsP, and/or a suitable material known in the art and theircombination at suitable thicknesses and doping concentrations. In oneparticular embodiment, the gain medium material 623 is an offsetmultiple quantum well (MQW) region gain chip that is flip chip bonded orwafer bonded or epitaxially grown across the “top” of one or moreoptical waveguides in the silicon layer of an SOI wafer.

In one embodiment where the gain medium material 623 includes activematerial, such as MQWs and with passive silicon waveguide based gratingsas reflectors or mirrors (at least one of which is a narrow surfacecorrugated grating), lasing may be obtained within the optical cavity622. In FIG. 6A, the lasing is shown with optical beam 619 reflectedback and forth between reflectors 607 and 609 in optical cavity 622. Inthe illustrated example, reflector 607 at the “back” side of the laserhas a higher power reflectivity than the reflector 609. Reflector 609 ispartially reflective such that optical beam 619 is output from a “front”side of the laser into the passive portion of the optical waveguidethrough which the optical beam 619 may be conducted to other components.

The reflective power of each of reflector 607 and 609 may be tuned basedon either or both of grating length and grating strength. In anembodiment, the power reflectivity of each of the reflectors 607 and 609is tuned independently based on the grating widths of the tworeflectors. In one embodiment, both the reflectors 607 and 609 arenarrow surface corrugated gratings to tune the grating strength bygrating width independently from grating depth. As an example, thereflector 607 is a first narrow surface corrugated grating with a firstgrating width less than the passive waveguide width while the reflector609 is a second narrow surface corrugated grating with a second gratingwidth less than the passive waveguide width, the second grating widthbeing the same or different than the first grating width. In one suchembodiment, reflector 607 and 609 each have a mirror length between 5and 500 μm and a grating width is less than 90% of the passive waveguidewidth which between approximately 1 and 1.5 μm.

In another embodiment, the reflector 609 is a narrow surface corrugatedgrating with a grating width less than the passive waveguide width whilethe reflector 607 is a grating with a grating width substantially equalto the passive waveguide width (i.e., only reflector 609 is a narrowsurface corrugated grating). In further embodiments, either or both thereflectors 607 and 609 may be an apodized narrow surface-corrugatedgrating, as described elsewhere herein.

FIG. 6B depicts an alternative embodiment where a DFB laser 602 isintegrated in an SOI substrate including a single crystallinesemiconductor layer 603 with a buried oxide layer 629 disposed betweenthe semiconductor layer 603 and a substrate layer 631. As furtherdepicted, an optical beam 619 is reflected back and forth betweenreflectors 607 and 609 within optical cavity 622. In particularembodiments, gratings are employed as reflectors 607 and 609 and thesegratings are disposed within the active portion of the silicon waveguideoptical cavity 622. In one embodiment, both the reflectors 607 and 609are narrow surface corrugated gratings for tuning the grating strengthby grating width independently of grating depth and grating length. Forexample, the reflector 607 is a first narrow surface corrugated gratingwith a first grating width less than the active waveguide width whilethe reflector 609 is a second narrow surface corrugated grating with asecond grating width less than the active waveguide width, the secondgrating width being either the same or different than the first gratingwidth.

In another embodiment, the reflector 609 is a narrow surface corrugatedgrating with a grating width less the active waveguide width while thereflector 607 is a grating having a width substantially equal to theactive waveguide widths (i.e., only reflector 609 is a narrow surfacecorrugated grating). In further embodiments, either or both thereflectors 607 and 609 may be an apodized narrow surface-corrugatedgrating, as described elsewhere herein.

FIG. 7 is an illustration of an exemplary optical system 751 utilizingintegrated optical components including an integrated semiconductormodulator multi-wavelength laser having an array of electrically pumpedhybrid semiconductor evanescent lasers 701 coupled to a passivesemiconductor layer over, on, or in, substrate 703. In one embodiment,each of the lasers in the laser array 701 may be an electrically pumpedhybrid silicon evanescent laser substantially as described withreference to FIGS. 6A-6B. In another embodiment, the laser array 701includes both a DBR laser utilizing a low grating strength and DFB laserutilizing a high grating strength. In the illustrated example, thesemiconductor substrate 703 of FIG. 11 is an optical chip that includesa plurality of optical waveguides 705A-705N over which a single bar ofgain medium material 723 is bonded to create an array of lasersgenerating a plurality of optical beams 719A-719N in the plurality ofoptical waveguides 705A-705N, respectively. The plurality of opticalbeams 719A-719N are modulated by modulators 713A-713N and then selectedwavelengths of the plurality of optical beams 719A-719N are thencombined in with optical add-drop multiplexer 717 to output a singleoptical beam 721, which is then to be transmitted through a singleoptical fiber 753 to an external optical receiver 757.

In an embodiment, at least one of the reflectors 709A-709N is a narrowsurface corrugated grating having a grating width less than thewaveguide width (passive and/or active depending on the type of laser)in which the grating is formed, as described elsewhere herein. In aparticular embodiment, each of the reflectors 709A-709N is a narrowsurface corrugated grating, one or more of which has a different widththan the others. In another embodiment, multiplexer 717 includes atleast one narrow surface corrugated grating having a grating width lessthan the width of optical waveguides 705A-705N in which the grating isformed, as described elsewhere herein. In other embodiments, any of thereflectors 709A-709N or multiplexer 717 may include an apodized narrowsurface-corrugated grating. As such, when any of the reflectors709A-709N and/or multiplexer 717 are simultaneously fabricated asoptical components integrated on semiconductor substrate 703,photolithography may be utilized to image a single photomask including aplurality of gratings, each having a width specified for the particulargrating power required for the particular application (e.g., laser,multiplexer filter, etc.).

In one embodiment, the integrated semiconductor modulatormulti-wavelength laser is capable of transmitting data at the multiplewavelengths included in the single optical beam 721 over the singleoptical fiber 753 at speeds of more than 1 Tb/s. In one example, theplurality of optical waveguides 705A-705N are spaced approximately50-100 μm apart in the single layer over semiconductor substrate 703.Accordingly, in one example, an entire bus of optical data is maybetransmitted from the integrated semiconductor modulator multi-wavelengthlaser with less than a 4 mm piece of substrate 703.

FIG. 7 also shows that in the example of optical system 751, the singlesemiconductor substrate 703 may also be coupled to receive an opticalbeam 721 from an external optical transmitter 759 through an opticalfiber 755. Therefore, in the illustrated embodiment, the singlesemiconductor substrate 703 is an ultra-high capacitytransmitter-receiver within a small form factor. While the opticalreceiver 757 and external optical transmitter 759 are also illustratedas existing on the same chip 761, it is appreciated that externaloptical receiver 757 and external optical transmitter 759 may beprovided on separate chips. In the illustrated embodiment, the receivedoptical beam 722 is received by an optical add/drop demultiplexer 718,which splits the received optical beam 722 into a plurality of opticalbeams 720A-720N. In one exemplary embodiment, the plurality of opticalbeams 720A-720N are split according to their respective wavelengths byone or more narrow surface corrugated gratings within the demultiplexer718 and are then directed through a plurality of optical waveguides706A-706N disposed in a thin film layer over, on, or in, semiconductorsubstrate 703.

As shown in the illustrated embodiment, one or more optical detectorsare optically coupled to each of the plurality of optical waveguides706A-706N to detect the respective plurality of optical beams 720A-720N.An array of photodetectors 763A-763N is optically coupled to theplurality of optical waveguides 706A-706N. As one example, each of thephotodetectors 763A-763N is a SiGe-based photodetector or the like. Inanother embodiment, also shown in FIG. 7, a single bar of semiconductormaterial 724 may be bonded to the a layer over, on, or in substrate 703across the plurality of optical waveguides 706A-706N to form an array ofphotodetectors optically coupled to the plurality of optical waveguides706A-706N. As one example, the single bar of semiconductor material 724includes III-V semiconductor material to create III-V photodetectors.With SiGe and III-V based photodetectors optically coupled to theplurality of optical waveguides 706A-706N as shown, a variety ofwavelengths for the plurality of optical beams 720A-720N may bedetected.

Control/pump circuitry may also be included or integrated onto thesubstrate 703. In one embodiment where the substrate 703 includes asilicon layer (e.g., SOI substrate), control circuit 762 may beintegrated directly in the silicon. In one example, the control circuit762 may be electrically coupled to control, monitor and/or electricallypump any of the lasers in the multi-wavelength laser array 701, theplurality of optical modulators 713A-713N, the arrays of photodetectors(e.g., 763A-713N) or other devices or structures disposed onto substrate703.

Thus, a narrow surface corrugated grating, method of manufacture andapplication in optical component integration has been disclosed.Although embodiments of the present invention have been described inlanguage specific to structural features or methodological acts, it isto be understood that the invention defined in the appended claims isnot necessarily limited to the specific features or acts described. Thespecific features and acts disclosed are to be understood merely asparticularly graceful implementations of the claimed invention providedin an effort to illustrate rather than limit the present invention.

1. A photonic device, comprising: a first and second passivesemiconductor waveguide region over a substrate; and an evanescentsemiconductor waveguide region coupled between the first and secondpassive waveguide regions, wherein one of the passive or evanescentwaveguide regions includes a first Bragg reflector comprising a surfacecorrugated grating with a width narrower than a width of the waveguideregion in which the grating is formed.
 2. The photonic device as inclaim 1, wherein the waveguide region including the first Braggreflector comprises two opposing sidewalls defining the waveguide widthat a top surface opposite the substrate; and wherein the surfacecorrugated grating comprises a plurality of grooves formed into thewaveguide top surface along a grating length, wherein a width of amajority of the grooves are narrower than the waveguide width byapproximately the same amount.
 3. The photonic device as in claim 1,wherein the waveguide region including the first Bragg reflectorcomprises two opposing sidewalls defining the waveguide width at a topsurface opposite the substrate; and wherein the surface corrugatedgrating comprises a plurality of teeth formed into the waveguide topsurface along a grating length, wherein a majority of the teeth arenarrower than the waveguide width by approximately the same amount. 4.The photonic device as in claim 1, wherein the semiconductor comprisessilicon, the first Bragg reflector has a mirror length between 5 and 500μm, the waveguide region including the first Bragg reflector has a widthof between approximately 1 and 1.5 μm, and the first grating width isless than 90% of the waveguide width.
 5. The photonic device as in claim1, wherein the semiconductor comprises silicon and wherein theevanescent waveguide region comprises an electrically pumped lightemitting layer disposed over the waveguide.
 6. The photonic device as inclaim 1, wherein the first passive waveguide region includes the firstBragg reflector and the second passive waveguide region includes asecond Bragg reflector forming a DBR laser, the second Bragg reflectorcomprising a surface corrugated grating with a second grating width thatis narrower than a width of the second passive waveguide region.
 7. Thephotonic device as in claim 6, wherein the first grating width isdifferent than the second grating width to provide the first gratingwith a higher grating strength than the second grating.
 8. The photonicdevice as in claim 1, wherein the evanescent waveguide region includesthe first Bragg reflector grating and a second Bragg reflector gratingto form a DFB laser, and wherein the first and second gratings each havea width narrower than the evanescent waveguide region.
 9. An apparatuscomprising a first photonic device as in claim 1; and a second photonicdevice as in claim 1, wherein the first and second photonic devices areintegrated onto the same substrate.
 10. The apparatus as in claim 9,wherein the first grating width of the first photonic device isdifferent from that of the second photonic device.
 11. A systemcomprising: a surface corrugated grating in an optical waveguide,wherein a width of the grating is narrower than a width of the opticalwaveguide, and wherein the grating is within, or at the interface of, anactive waveguide portion of a laser; and an optical modulator coupled tothe optical waveguide to modulate the light generated by the laser. 12.The system as in claim 11, wherein the optical waveguide comprisessilicon and wherein the grating defines an optical cavity of a hybridsilicon evanescent laser.
 13. The system as in claim 12, wherein thegrating comprises a plurality of grooves in a top surface of the opticalwaveguide, and wherein a width of a majority of the grooves is narrowerthan a width of the waveguide top surface by approximately the sameamount.
 14. A narrow surface corrugated grating, comprising: an opticalwaveguide over a substrate, the waveguide including two opposingsidewalls defining a width of a top surface opposite the substrate; anda plurality of teeth and a plurality of grooves there between formedinto the waveguide top surface along a grating length, wherein a widthof substantially all of the grooves or teeth is narrower than thewaveguide top surface width.
 15. The corrugated grating as in claim 14,wherein the groove or tooth width is approximately equal across amajority of the plurality and is no greater than 90% of the waveguidetop surface width.
 16. The corrugated grating as in claim 14, whereinthe waveguide comprises silicon, the waveguide top surface width isbetween approximately 1 and 1.5 μm, the grating length is betweenapproximately 1 and 200 μm, and the grating has a pitch betweenapproximately 190 and 250 nm.
 17. The corrugated grating as in claim 14,wherein a subset of the plurality of grooves have a varying groovewidth, apodizing the narrow grating.
 18. A method of forming a narrowsurface corrugated grating, comprising: patterning, based on a firstmasked lithographic exposure, a plurality of grating grooves and aplurality of grating teeth there between into a surface of a thin filmto corrugate the surface; and patterning, based on a second maskedlithographic exposure, a waveguide into the thin film, the waveguidehaving two opposing sidewalls demarking a waveguide width, wherein thecorrugated surface is disposed between the waveguide sidewalls to form aportion of a waveguide top surface, wherein the width of substantiallyall of the grooves or teeth is less than the waveguide width.
 19. Themethod as in claim 18, wherein the patterning of the grating and thepatterning of waveguide each further comprise: depositing aphotosensitive layer over the substrate; exposing, through a photomask,the photosensitive layer to electromagnetic radiation; and developingthe photosensitive layer after the exposure to form an etch maskpattern.
 20. The method as in claim 19, wherein the electromagneticradiation has a wavelength of approximately 193 nm, the waveguide ispatterned to a width between approximately 1 and 1.5 μm and the surfacecorrugated grating is patterned to a length of between approximately 1and 200 μm, a pitch of between 190 and 250 nm and a width of betweenapproximately 0.3 μm and 1.3 μm.